Electroacoustical apparatus and method of using the same



4 Sheets-Sheet l A S S A M F Filed 001'.. 31, 1959 r v. u w 1! A ELECTROACOUSTICAL APPARATUS AND METHOD OF USING THE.:SAME

arch 12, 1946. l F. MASSA j ELECTROACOUSTICAL APPARATUS AND METHOD OF USING THE SAME Filed Oct. 3l, 1939 4 Sheets-Sheet 2 F. MASSA 2,396,519

ELECTROACOUSTICAL APPARATUS AND METHOD OF USING THE SAME March l2, 1946.

4 Sheets-Sheet 3 Filed Oct. 51, 1939 F. MASSA March l2, 1946.

ELECTROACOUSTICAL APPARATUS AND METHQD OF USING THE SAME 'led Oct. 3l, 1959 4 Sheets-Sheet 4 W W V w,

Patented Mar. l2, 1346l UNITED STATES PATENT OFFICE ELECTROACOUSTICAL APPARATUS AND METHOD F USING THE SAME Frank Massa, Audubon, N. J., assigner to Radio Corporation of Americaa corporation of Dela- Ware Application Gctober 31, 1939, Serial No. 302,192

7 Claims.

This invention relates to electroacoustical apparatus and the method of -employing the same.

and particularly to a microphone of the pressureprovide an improved microphone of the type set forth which may be used as an absolute labora-l tory standard at the higher audio frequencies.

Still a further object of my present invention is to provide improved means for accurately loeating a source of sound with a microphone of compact design.

It is also an object of my present invention to provide a novel method of guiding ships through fogs, locating airplanes in night, etc., by means of a microphone' having compact design, high sensitivity, and high, as Well as controllable, directional selectivity.

In accordance with my present invention, I provide a microphone which consists essentially of two diaphragms mounted on opposite ends of a sealed cylindrical tube or casing. Both diaphragms are mechanically connected to and drive the same voice coil, the difference in pressure acting on the two diaphragms causing thc operation of the microphone. By mounting one or more of these microphones at a given point and orienting them relative to a sound source which is to be located until a predetermined re spouse characteristic is obtained, it is possible to accurately locate the sound source. The combined high sensitivity and high directional-selectivity of my improved microphone and microphone assembly, together with the compact; mechanical structure embodied therein, provides one of the main advantages of my present invention over microphones and sound detecting devices heretofore known.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims, The invention itself, however, both as to its organization and method of operation, as well as additional obiects and advantages thereof,` will best be un' derstood from the following description, when read in connection with the accompanying drawings, in which AFigure 1 is a central' sectional view of one form of microphone according to my present invention,

Figure 2 is 'a sectional view thereof taken on the line II-II of Figure 1,

Figure 3 is a composite view showing a schematic representation of a microphone according to my present invention and a curve showing the maximum pressure differences between the two diaphragms thereof,

Figures 4 and 5 are sensitivity curves showing the response that may be obtained from my improved microphone,

Figures 6 to 10, inclusive, show directional characteristic-s of my improved microphone atY various wave lengths,

Figure l1 is a, view showing one way of einploying a single microphone according to my invention for locating a sound source,

Figure 12 is a similar view showing how a pair of microphones according to my present invention are employed for th'e same purpose,

Figure 13 is a diagrammatic view with rcference to which a quantitative analysis is given hereinafter to show how the directional characteristics of the assembly of Fig. 12 vary with spacing and frequency,

Figure 14 is a vector diagram showing the voltages induced in each of the microphones shown in Fig. 13,

Figure 15 shows a family of characteristic curves pertinent to the system illustrated in Fig. 13,

Figure 16 is a plan view of a sound detecting system employing two microphones according to my present invention, the microphones being so arranged that the distance between their axes may be varied,

' Figure 1'7 is an end elevation thereof Figure 18 is a perspective view of a'` different assembly employing a plurality of microphones according to my present invention and arranged for sound detection,

Figure 19 is a plan view showing still another arrangement for this purpose, and

Figure 20 is a side elevation of the latter form of my invention.

Referring more particularly to the drawings.

' wherein similar reference characters indicate corresponding parts throughout, I have shown, in Figs. 1 and 2, a cylindrical tube or casing i which is normally open at its ends, but which is sealed portion 3a, an annular rim or peripheral portion l 3b and an intermediate flexible-portion 3c, the

diaphragms 3 each being securely clamped to the ends of the cylindrical casing I by means of a clamping ring 5,.,between which and the casing I the peripheral portion 3b is confined, and a plupole piece, and a core I1 which constitutes an inner pole piece, the pole pieces I5 and I1 being separated from each other to provide an annular air gap I 9. Movably mounted in the air gap I9 is an annular conductive coil 2| which is vconnected to the diaphragms 3 and is supported within the air gap I9 by a plurality of thin, elongated rods or tubesl 23. The rods or tubes 23 are Vfastened rigidly to the voice coil II9 in any suitable manner and are spaced circumferentially about the voice coil. Those rods 23 which are in the upper half of the microphone, as shown in Fig. l, pass through a series of circumferentially spaced openings Sd in the associated diaphragm 3 and the ends thereof are sealed to the diaphragm by cement or'the like. The lower set of rods 23, as viewed in Fig. l, are also securely fastened to the `voice coil 2i and pass through the openings II, with the ends thereof extending through circumferentially spaced openings 3d in the lower diaphragm 3 where they are similarly sealed to the latter diaphragm. A cover plate Ia which fits over an opening Ib in the casing I for a purpose shortly to be set forth carries a pair of terminals 25 to which the leads from the voice coil 2l are connected so that external connection of the voice coil may be made to a suitable amplifier.

The rods 23 are preferably made of magnesium aluminum alloy for minimum'weight and maximum strength, and-in the case of .large structures, which are advantageous for low frequency applications, the supports 23 may be in the form of hollow tubes. In either case', to assemble the voice coil I9 and the supporting members 23, the 'latter are preferably located in a jig which will insure accurate alignment of the voice coil 2| with respect to the free ends'of the supporting rods 23. After the assembly ot the coil and the supporting rods is complete, it mayv be assembled into the casing I as follows: One of the diaphragms 3 (for example, the upper one as viewed in Fig. 1) is removed from its mounting and the magnet structure` and the other diaphragm are mounted in place. The coil assembly, including the rods 23, are then inserted into the air gap through the upper, open endof the casing I, the coil supports 23 being passed through the clearance openings l I in the plate 9 and through the openings 3d in the lower diaphragm 3. Shims are then placed inside and outside voice coil diameters so that the proper clearance thereof in the air gap lI9 may be determined. With the shims in place, the, coll 2! is slightly rotated until the rods 23 are all free within the diaphragm clearance holes 8d and also the openings II. It is for this purpose that the opening 'Ib may-be provided in the casing i, so that easy access may be had to the air gap. Havl ing located the coil 2l in theair gap I9 and the lower rods 28 in the clearance holes 3d of the lowerdiaphragm 3, the upper diaphragm 3 isl then assembled onthe upper end of the cylindrical 75 casing I, taking care that the upper rods 23 lie free within the clearance holes Sei-provided for them in the upper diaphragm. After both diaphragms are assembled in place, cement may be applied to the protruding ends of al1 of the rods v23, as heretofore noted, to seal the rods to their respective diaphragms. The cement having set, the shims are then removed from the air gap I9 and the coil 2l is left free withinthe air gap I9. The leads from the voice coil may then be secured to the terminals 25 and the cover plate Ia secured in place over the opening ib by means of screws or the like 21. l

When such a microphone is placed in a sound field, a driving force will be exerted on the voice coil 2l equal to the difference in pressure acting on each diaphragm 3 multiplied by the diaphragm area. If a sound wave arrives along the axis normal to the diaphragms, and if 'the cylinder I is a small vfraction of 'a wavelength in all dimensions, the driving force is proportional .to the pressure gradient of the sound wave. If the vibrating system consisting of the voice coil 2i and diaphragms 3 is Ymass-controlled (thatis, if the frequency oi' the sound wave is sufiiciently higher than the natural frequency of the vibrating system such that the mechanical impedance is substantially directly proportional to the frequency), then the voltage induced in the voice coil 2l will be independent oi the frequency. y

The quantitative relations between sensitivity and separation between diaphragms will now be derived so that information will be available for designing microphones of maximum sensitivity to operate over predetermined portions of the frequency range.

In Fig. 3 is showna schematic representation of a pressure gradient microphone according to my present invention and including two innitesimally small pistons X and Y corresponding to the diaphragme 3, the piston Xand Y being separated by a distance d. A plane wave of wave length A is shown arriving at en angle o with the axis of the cylinder I, and the position of the wave ischosen to show the maximum instantaneous pressure difference acting on the system.

For sounds arriving at angle 0:0, the instantaneous pressureacting on piston X is P0x=f m., sin @x 180) v(1) and the instantaneous pressure acting on piston Yis - yThe total maximum instantaneous force acting on the mechanical system is Fm(.o)=2Pm sin (X 180)- where A is the diaphragm area, and Pmx is the maximum instantaneous sound pressure.

For sound arriving at an angle 0 (Fig. 3) with the axis of the cylinder I. it can be similarly (Liegi-gfx (4) 'l shown that FmnxG-M :2A Pm sin For a mass-controlled system, the mum instantaneous velocity of the voice coil for sounds arriving at 8:@ is'equal to Y.

`assimile in which mo=total eifectivevibratng mass, and f=requency of the sound wave.

x"mur (9:0) I: sin 180D) where K- 21rcm0 The open circuityoitage induced in the voice coil 2| is proportional to the Velocity of the voice coil. Therefore, Equation '7 represents the sensitivity of the-microphone as a function of d and i (assuming the diaphragm area A is very small compared to thewave length of sound).

Two sensitivity curves were computed from' Equation '7 and are shown in Figs. 4 and 5. For Fig. 4, the microphone length d was kept constant and the frequency of sound was allowed to vary.

' The relative response characteristic as a function of d/ (d kept constant and A varied) is plotted in Fig; 4. These data show that uniform response is not obtained with frequency if the wave length becomes an appreciable fraction of the microphone length d. The choice of length d for uniform response should be governed by the highest frequency to be reproduced; and at that frequency, choose d/ from Fig. 4 such as to give whatever drop in response may be permitted in the system. For example, if A10 percent drop may be permitted at highest frequency of operation, d/A should not be greater than 0.25 for that frequency.

In Fig. 5, there is. plotted another sensitivityA curve, also computed from Equation 7, in which the relative sensitivity at a fixed frequency (i kept constant) is shown as d is varied. From this curve, it appears that the sensitivity is approximately directly proportional to'd for frequencies below those which made d/lr equal to x15. At higher frequencies, the sensitivity increases more slowly with increasing length d, reaching a maximum at d/ \=1/2. By an inspection of Fig.' 5, it is apparent how much increase in sensitivity is possible if a microphone is only to be used for low frequency work. For example, a microphone intended for use over the greater part of the audio frequency range should be designed with d equal to a fraction of an inch. If such a microphone is employed for low' frequency use d/k will be o f the order of 1/100, for which the relative sensitivity is only 3 percent, as seen from Fig. 5. By increasing d, it is obvious that the sensitivity may be increased many times in the low frequency region.

Another means may be simultaneously employed in a low frequency microphone for further increasing the sensitivity,"namely, the diaphragm areas may be greatly increased. The assumedv condition of an infinitesimally small diaphragm is approximately satised for diaphragm diameters less than 116 wavelength of the sound wave,

which means that the diameter may be several inches at the lower frequencies with the corresponding increase in sensitivity which such increase in area willpermit.

To build a pressure gradient microphone for use at the very high frequencies, it is only necest finite size,

sary to make all the dimensions small enough to satisfy the above mentioned relations for the highest frequency concerned. It is obvious that the natural resonance frequency of such a tiny structure will be inherently rather high, which means that the microphone will not be mass controlled at the lower frequencies. Uniform response will therefore occur at frequencies above and removed from the resonance frequency of the system. Such a microphone will operate satisfactorily as a high frequency instrument which may be used in the frequency range above 1000 cycles and may bemade a precision instrument to extend into the supersonic range; (The highest frequency of operation lwith uniform characteristics is limited only by how small the unit may be built, which is governed by theV mechanical skill ofthe maker.)

The quantitative relations between sensitivity and microphone dimensions have been deduced above; and now the directional response-characteristics of the microphone as a function of the frequency and dimensions will be derived. The directional response of a Vmicrophone shows the relative response of the microphone to. sounds arriving along various axes of the structure. Looking back at Equations 3 and 4, we see the expressions for the driving forces acting on the microphone for a plane wave arriving along an axis 0:0 and 0:0 (Fig. 3). The relative response at angle 0 is given by the ratio of Equation 4 over Equation 3, which gives Response at 0 sin i-c-s-X 180) (8) Response at 0 sin (LXISW) An inspection of Equation 8 shows that "for very small values of d/ the equation reduces to simply cos 0. At increasing values of d/A, the direction-a1 response characteristics become broader than the true cosine characteristics. Figs. 6 to 10, inclusive, show the directional patterns for various values of d/ indicated in the drawings and as computed from Equation 8. Comparing Fig. 4 with Fig. 5, it is apparent that the directional characteristics become broader as the response on the axis decreases with increasing frequencies.

In all the data which have been presented showing the relations between the microphone size and its various characteristics, it has been assumed thatthe diameterof the diaphragms 3 is very small. If the diaphragm diameter becomes appreciable, the computed characteristics will be somewhat modified, due to the variation in pressure at the diaphragm surface due to its For diaphragm diameters less than #e the wavelength of sound, such diffraction effects may be neglected without appreciable error. If the diameter is greater than 116 wavelength, the pressure on the diaphragm on which aplane wave isv directed will increase over the free wave pressure with increasing frequency, becoming double the free wave pressure at some frequency for which the diaphragm diameter is greater than the wavelength of sound.

Thus, if, for the response curve of Fig. 4 and the sensitivity curve of Fig. 5, the diaphragm diameter is greater than ls wavelength at any value of d/i, the values shown by the ordinates will, in general, have to be multiplied by a factor greater than unity. As far as the response curve is concerned, if the diaphragm diameter becomes greater than fai for values of d/ greater than 0.1, there will be a tendency for the response curve to .remain attcr than shown by Fig. 4. Also, the broadening effect shownby Figs. 6 to 10. inclusive, will be sharpened for some of the values of d/i. Thus, although it is preferable ings" on a sound beacon which islset up at a fixed location for the purpose of guiding ships when visibility is low. Another application for my in.. vention is to locate airplanes in flight from the noise generated by the propeller or engine exhaust. For this latter application, the apparatus will be much lighter and less bulky than the heavy and cumbersome four-horn arrangement which is now being employed.

The simplest arrangement employing this in.- vention is indicated schematically in Fig. 11.

Here, an ultra sensitive pressure gradient microphone 3| is rotatably mounted at some convenangie 0 with the microphone axes.

ient part of a ship 32 and arranged so that the microphone axis may be varied with respect to the axis of the ship. To take a bearing on the sound source, the microphone 3i is rotated on its support until zero response is indicated. Telephone receivers or electric meters may be employed as an indicator and an amplifier may or may not be provided with the microphone, de-

" pending on its sensitivity and degree of precision desired. Fig. l1 shows the microphone oriented to indicate the location of a sound beacon 35, the location of which is to be determined. Angular location may be read from 'a suitable protractor head 36 assembled on the microphone mounting. Either a manual or automatic means may be provided for turning the microphone 3i, and remote control and indicating. means may be employed by using flexible shafts or Selsyn motors.

Although the arrangement of Fig. 11 offers a n very simple and accurate means for obtaining sonic bearings -under certain conditions, it will lose precision if the ship is in the neighborhood of a waterfront having aseries of tall buildings which reflect the sound. Under these conditions,

it may be practically impossible to get a zero reading for the microphone setting.

An alternative method for detecting a sound beacon is shown in Fig. 12. In this case, two microphones 3i and M of the same sensitivity and characteristics are mounted with their axespari allel and separated, and theirelectrical outputs preferably connected in series, although they may be connected in parallel relation, if desired. Such Aan arrangement 'may be made to give a very sharp directional characteristic which permits the location of the sound beacon 34 from the orientation of the microphone assembly which gives an indication of maximum output. The arrangement of Fig. 12 will, in general, be free of reflection errors mentioned above, and, because' it operates at maximum output.' it requires va less sensitive detecting arrangement than that of Fig.

1l; or, for the same detector sensitivity, it may aseaie be used at greater distances from the sound source. v f

In order to permit the proper spacing between microphones, a quantitative analysis will be given to show how the directional characteristics of the microphone combination vary with spacing and frequency. For this purpose, reference is now made to Fig. 13, which shows the two pressure gradient microphones 3i and 4I arranged with their axes parallel and separated by a distance D. A plane wave is shown arriving at an Both microphones 3l and 4I are assumed to have true cosine characteristics, as shown by the dotted circles on each microphone, and both microphones are assumed to be of equal sensitivity.

The vector diagram shown in Fig. 14 shows the voltages induced in each microphone, en and en, and also their sum, eo. From an inspection of Fig. 14, it is obvious that the absolute magnitudes l of the voltages esi and en are equal, and the following relations may be written:

The phase angle, o, between the voltages en and en iS equal to Dsin The voltage eo in Fig. 14 is equal to 11) For sound arriving at 0=0 where D=distance between microphone axes, and A=wavelength of sound.

Equation 13 shows the directional characteristic of the microphone combination of Fig. 13 as a function of frequency and separation. If D is very small compared to i, Equation 13 becomes equal to simply cos 0, which is the same characteristic as a single microphone. As D is increased, however. the directional characteristic of the combination becomes sharper. The shape of the characteristic for various values of D/i indicated in this ligure, and as computed from Equation i3, is shown in Fig. 15. From this information. it is possible to so separate the microphones in Fig. 12 as to obtain any degree of sharpness that is desired for locating the sound beacon 34.

For certain applications, it may be desirable to have the'microphones` close together to give a broad pick-up characteristic. in which case the sound source may be readily located, and then two separate the microphones for accurate "spotting' of the source. Such an arrangement may be particularly desirable for airplane detection, for example. Several methods may be employed for. separating the microphones. For example, they may be mounted on horizontal tracks and displaced thereon as desired, either manually or by auxiliary mechanical means. Another method for varying the microphone separation is shown in Fig. 16. Here the two microphones 3| and 4| are mounted, respectively, on the supports 33 and 43 which are fixed to a pair of beveled gears 35 and 45 and which may be swung in a fan-shaped manner, as indicated in Fig. 17. In the driving arrangement shown, the two beveled gears 35 and 45 are in mesh with a common pinion 31 which is arranged to drive the gears 35 and 45 in opposite directions, thus keeping the microphone positions at all times symmetrical with respect to the vertical axis. A worm and wheel arrangement 39 may be employed to operate the driving pinion so that the microphones 3| and 4| will remain xed in any position at which they may be set. Of course, any other mechanical driving arrangement may be employed and braking means provided to keep the microphones from falling under the action of gravity. The complete microphone mounting is preferably fixed to a pivoted base 40 to permit orientation of the structure for determining the angular position of the sound source with respect to some xed axis of reference.

Fig. 18 shows a schematic representation of thecombination of four ultra sensitive pressure 'gradient microphones according to this invention for the purpose of locating aircraft in flight. Two pairs of microphones a, 5|b and 53a, 53h are each so arranged that the two sets of microphones may be independently rotated in right angle planes. For this purpose, the four microphones are supported on tubular sleeves 54 which are slidably and rotatablyA carried on armsf55 secured to 'the ball member of a ball and socket mounting 51, so that each set may be oriented independently of the other, each to give maximum output indication. The intersection of the two planes, one containing the microphones 5|a and 5|b and the other containing the microphones 53a and 53h will show the axis along which the airplane is located. Since the two sets of microphones of Fig. 18 are mounted on the arms 55, the separation of each pair of microphones in the plane thereof may then be varied to provide any desired degree of i sharpness of location, as noted above. It is obvious that the arrangement of Fig. 18 is preferably operated by two persons, one to locate one and other to locate the other axis. Each person follows the ,planes progress independently, and the intersecting axes of the two planes of the microphones 5|a and 5|b, on the other hand, and 53a and 53h, on the other, indicate the planes positionu from instant to instant. It is also obvious, of course, that each pair of microphones may be mounted on a separate universal mounting 51 for independent adjustment.

A modication of the system shown in Fig. 18 which may be operated by one person is shown in Figs. 19 and 20.. In this arrangement, a plurality of sets of microphones Sla, lb'; 63a, 63h; 65a, 65h; and 61a, 61D are shown arranged on a circle around a shaft or axis 69, the shaft 69 having a v universal mountingll on a suitable standard '13.

The axesof the? microphones are all parallel to each other and the outputs of all the microphones (Whichare of equal sensitivity) are preferably connected in series. Each of the microphones is carried' on a bracket 15 slidably carried on a series of diametrically opposed, radial arms 11 on the shaft 69. Each of the brackets has fixed thereto a downwardly extending pin which extends through a' slot 'I9 in its associated arm 11 and also through an associated arcuate slot 8| formed in a disc 83 which is rotatably mounted on the shaft' G9 and which is provided with an operating handle 85. As the handle is manipulated to rotate the disc 83, it is obvious that the arcuate slots 8| acting upon the pins 8U, will cause the microphones to move radially along the arms 'H Within the slot 'I9 thereof, either toward or away from the shaft 69, depending upon the direction of rotation of the disc 83. It will be noted, however, that all of the microphones remain fixed in regard toy their axial parallelism, that is, their respective axes always remain parallel to each other and to the common axis 69. The directional response characteristic of such an arrangement of microphones may be made,

very sharp with respect to the axis of symmetry 59 by suitable microphone separation by means of the disc 83, and a single person may operate the structure to follow aircraft in night. The circular arrangement of microphones will permit the spotting of an airplane much like a spotlight would be used.

The arrangement of Figs. 19 and 20 maybe further modied, if desired, to combine the principles of the modification shown in Fig. 18 with that shown in Figs. 19 and 20. If the mechanical arrangement of the modication of Figs. 19 and 20 is made such that opposite pairs of microphones may be independently connected and moved similar to the arrangement of Fig. 18, it is obvious that the portion of the modification of Figs. 19 and 20 thus employed becomes identical to that shown in Fig. 18. .Then, by reconnecting the structures as a unitary arrangement of parallel, circularly disposed elements or microphones, the` apparatus is again the acoustic spot-light of Figs. 19 and 20. l

From the foregoing description, it will be apparent to those skilled in the art that I have provided a simple and compact mircophone of the pressure gradient responsive type which has great sensitivity and the characteristicsV of which may be easily controlled. It will also be apparent that my improved microphone can b'e used effectively to locate a source of sound with great accuracy. Although I have shown and described but one specific embodiment of the microphone together with several ways in which it can be used, I am fully aware that there are many other modifications thereof possible and also lthat it may be used in many different ways for the purposes set forth, as well as for other purposes. I, therefore, desire that my invention shall not be limited except insofar as is made necessary by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. A pressure gradient responsive microphone comprising a hollow casing normally open at its ends, a diaphragm at each end of said casing sealing off the interior of said casing from Athe atmosphere, a field structure within said casing, said eld structure comprising a supporting plate and a pair of pole pieces carried by sail plate in spaced relation to each other to provide an air gap therebetween, said plate having a plurality of openings extending therethrough in circumferentially spaced relation, a conductive coil in said air gap, and a plurality of thin, elongated members connecting each of said diaphragms to said coil whereby vibration of said diaphragms in response to acoustical energy moves said coil in said air gap in accordance with the difference in pressure applied to the respective diaphragms by said acoustical energy, said members being spaced circumferentially about said coil and said diaphragme, and certain of said members passing through said openings.

2. A pressure gradient responsive microphone comprising a hollow casing normally open at its ends, a diaphragm at each end of said casing sealing on the interior of said casing from the atmosphere, a field structure within said casing, said eld structure comprising a supporting plate and a pair of pole pieces therein spaced from each other and arranged to provide an annular air gap therebetween, said plate having a plurality of openings extending therethrough in circumfer-` entially spaced relation, an annular, conductive coil in said air gap, and a plurality of thin, elongatedmembers connecting each of said diaphragms to said coil, whereby vibration of said diaphragme in lresponse to acoustical energy moves said coil in said air gap in accordance with the dierence in pressure applied to the respective diaphragms by said acoustical energy, said members being spaced circumferentially about said coil and said diaphragms, certain of said members passing directly from said coil to one of said diaphragme, and certain other of said membersv passing from said coil to the other of said diaphragme through said openings.

` 3. The invention set forth in claim 2 characterized in that said casing is cylindrical in form.

4. The invention set forth in claim 2 characl terized in thai-.said casing is cylindrical in form,

characterized further in that each of said diaasoasio phragms is constituted by a circular member having a relatively stiff central portion, an annular peripheral portion by which the diaphragms may be secured in place, and an intermediate flenible portion, characterized further by the addition of means for clamping said diaphragms securely to said casing at said peripheral portions whereby said central portions are adapted to act substantially as pistons, and characterized still further in that said elongated members are secured to said central portions of the respective diaphragms.

5. The invention set forth in claim 2 characterized in that said eld structure is located substantially centrally of said casing.

6. The invention set forth in claim 2 characterized in that said casing is provided with an opening through which access may be had to the interior thereof. and characterized further by the addition of a cover plate which is secured to said casing over said opening. l

' '1. The invention set forth in claim 2 characterized in that said casing is provided with an opening through which access may be had to the interior thereof, characterized further by the addition of a cover plate which is secured to said casing over said opening, and characterized still further by the addition of conductive terminals on said cover plate to which the leads from said coil are connected.

FRANK MASSA. 

